Rab11A regulates dense granule transport and secretion during Toxoplasma gondii invasion of host cells and parasite replication.

Kannan Venugopal,Javier Periz,Nicolas Barois,Markus Meissner,Sabrina Marion,Isabelle Tardieux,Elisabeth Werkmeister,Frank Lafont,Sylia Chehade,Gordon Langsley,Jamal Khalife

doi:10.1371/journal.ppat.1008106

Abstract

Toxoplasma gondii possesses an armada of secreted virulent factors that enable parasite invasion and survival into host cells. These factors are contained in specific secretory organelles, the rhoptries, micronemes and dense granules that release their content upon host cell recognition. Dense granules are secreted in a constitutive manner during parasite replication and play a crucial role in modulating host metabolic and immune responses. While the molecular mechanisms triggering rhoptry and microneme release upon host cell adhesion have been well studied, constitutive secretion remains a poorly explored aspect of T. gondii vesicular trafficking. Here, we investigated the role of the small GTPase Rab11A, a known regulator of exocytosis in eukaryotic cells. Our data revealed an essential role of Rab11A in promoting the cytoskeleton driven transport of dense granules and the release of their content into the vacuolar space. Rab11A also regulates transmembrane protein trafficking and localization during parasite replication, indicating a broader role of Rab11A in cargo exocytosis at the plasma membrane. Moreover, we found that Rab11A also regulates extracellular parasite motility and adhesion to host cells. In line with these findings, MIC2 secretion was altered in Rab11A-defective parasites, which also exhibited severe morphological defects. Strikingly, by live imaging we observed a polarized accumulation of Rab11A-positive vesicles and dense granules at the apical pole of extracellular motile and invading parasites suggesting that apically polarized Rab11A-dependent delivery of cargo regulates early secretory events during parasite entry into host cells.

Highlights

Toxoplasma gondii (T. gondii) is an obligatory intracellular parasite that belongs to the phylum Apicomplexa, typified by the presence of specific apical secretory organelles called rhoptries and micronemes
Toxoplasma gondii (T. gondii) is a highly prevalent parasite infecting a wide range of animals as well as humans
T. gondii secretes numerous virulent factors contained in specific organelles, termed the rhoptries, micronemes and dense granules

Summary

Introduction

Toxoplasma gondii (T. gondii) is an obligatory intracellular parasite that belongs to the phylum Apicomplexa, typified by the presence of specific apical secretory organelles called rhoptries and micronemes. Upon contact with the host cell, rhoptry (ROP) and microneme (MIC) proteins are released in order to promote parasite entry by driving the formation of a tight parasite-host cell adhesive membrane structure (termed the moving junction) [1]. The molecular mechanisms regulating MIC exocytosis have been well studied leading to the discovery of specific parasite signaling pathways triggering their secretion upon parasite adhesion to host cells [2] [3]. In contrast to micronemes and rhoptries, DGs are randomly distributed in the parasite cytosol and the mechanisms regulating their exocytosis at the parasite plasma membrane (PM) have not been elucidated. A recent study reported that in contrast to microneme exocytosis dense granule release is negatively regulated by cytosolic Ca2+ [9].

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